Zindīq (Arabic: زنديق) (from Persian Zendik (Persian: زنديک) meaning free interpretation, free thinker, atheist or heretic) is applied by Muslims to individuals who are considered to hold views or follow practices that are contrary to central Islamic dogmas. Starting in medieval times, Muslims began to refer to Manichaeans, apostates, pagans, heretics and those who antagonized Islam as zindiqs, the charge being punishable by death. As of the late 8th century the Abbasid caliphs began to hunt down and exterminate freethinkers in large numbers, putting to death anyone on mere suspicion of being a zindiq. They were extensively persecuted on an organized scale starting in the reign of al-Mahdi and then continued by his successors, al-Hadi and Harun al-Rashid, although with diminished intensity during the reign of the latter. Starting with al-Rashid's successor al-Ma'mun, religious persecution in Islam took a different direction with the institution of Mihna.
In modern times Zindīq is occasionally used to denote members of religions, sects or cults that originated in a Muslim society but are considered heretical or independent faiths by mainstream Muslims.
The word zendiq is now known to have derived from Middle Persian Pahlavi word of zandik or zendik (Persian: زنديك) consisting of zand plus îk (attribution suffix in Pahlavi language) referring to those who resorted to interpretation in their understanding of Zoroastrian faith. According to Dehkhoda Persian Dictionary zand is derived from Avestan zanda found in two instances in Avesta  whose root is unknown today, however it has seemingly implied sinners such as bandits, thieves, enchanters, renegades and liars. A different, common view on the etymology of the term is that it alluded to "free interpretation" or "commentary" on the sacred texts, with the same root that occurs in the word Zand, referring to the commentary on the Avesta (compare the Muslim term Batiniyya). The first recorded use of the word zandik is probably on the inscription in Naqsh-e Rajab attributed to Kartir, high-priest and advisor of Sassanid emperors Hormizd I, Bahram I and Bahram II, in which it explicitly denotes Manichaeans as "the ones with corrupted faith".
Famous and alleged zendiqs in Islamic history
- Abu Nuwas
- Muhammad ibn Zakariya al-Razi
- Ibn al-Khatib
- Ibn al-Muqaffa'
- Abu Shakir
- Abu Tammar Muttabib 
- Muhammad al Warraq 
- Ibn al-Rawandi 
- Yazdan ibn Badhan (Persian: یزدان پور باذان)
- Bashar ibn Burd
- Abdulkarim ibn abi Al-Ouja'
- Khalid al-Qasri
- The new encyclopedia of Islam By Cyril Glassé, Huston Smith P491 "Zindiq from Persian zand. ‘free interpretation” meaning “heresy’). A freethinker. atheist, or heretic."
- Islam in history: ideas, people, and events in the Middle East By Bernard Lewis p287 "In legal parlance the Zindiq is the criminal dissident—the professing Muslim who holds beliefs or follows practices contrary to the central dogmas of Islam and is therefore to be regarded as an apostate and an infidel. The jurists differ as to the theoretical formulation of the point of exclusion, but in fact usually adopt the practical criterion of open rebellion."
- JOHN BOWKER. "Zindiq." The Concise Oxford Dictionary of World Religions. 1997
- The new encyclopedia of Islam By Cyril Glassé, Huston Smith P491 "Tolerance is laudable, "The Slaughterer" the Caliph Abu al-`Abbās had once said; except in matters dangerous to religious belief, or to the Sovereign’s dignity. Al-Mahdi. (785) persecuted Freethinkers, and executed them in large numbers. He was the first Caliph to order the composition of polemical works in refutation of Freethinkers and other heretics; and for years he tried to exterminate them absolutely. hunting them down throughout all provinces and putting accused persons to death on mere suspicion."
- Muhammad Qasim Zaman (1997). Religion and Politics Under the Early ʻAbbāsids: The Emrgence of the Proto-Sunnī Elite. BRILL. p. 64. ISBN 978-90-04-10678-9.
- Zarrinkoub, Abdolhosein (1999), Two Centuries of Silence, ISBN 964-5983-33-6
- Yasna 61, 3; Vendidad 18, 53-55.
- Peters, Francis E. 2003. The Monotheists: The peoples of God. P.194
- Glassé, Cyril and Huston Smith. The new encyclopedia of Islam . P.491
- Schimmel, Annemarie. 1992. Islam. P.72
- Ṭabarī, Clifford Edmund Bosworth. 1999. The Sāsānids, the Byzantines, the Lakhmids, and Yemen. P.38
- Monnot, Guy. 1974. Penseurs musulmans et religions iraniennes: ʻAbd al-Jabbār et ses devanciers. P.98
- Colpe, Carsten. 2003 Iranier, Aramäer, Hebräer, Hellenen : iranische Religionen und ihre Westbeziehungen : Einzelstudien und Versuch einer Zusammenschau. P.129
- Dehkhoda Persian Dictionary.
- Jennifer Michael Hecht, "Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas Jefferson and Emily Dickinson", pg. 227-230
- [Ali] (1978), Hallaj (in Persian) (10th ed.), Germany: Alborz, pp. 102–126; Chapter on Zendiqs and Materialistic Thinkers.
- Awasaf an-Nas fi Tawarikh wa Silat, pp19, Mohamed Kamal Chabana